This invention relates to aqueous boron-containing compositions and their preparation.
Borate-containing compositions have various applications in industry, e.g. in the vitreous, agriculture and wood preservatives fields as well as in fire retardant applications. Conventionally the borate has been provided in granular form and, particularly for application by spraying, it has been necessary to dissolve the granules, requiring heating before application.
Aqueous borate suspension formulations have been proposed for the treatment of timber. Thus AU-A-574389 proposes the use of a preservative composition comprising a colloidal micro-crystalline suspension of boron salts and a thickening agent, which serves to maintain the stability of the suspension. The particles of the colloidal micro-crystalline suspension are observed to be of the order of 15 to 30 microns. Similarly EP-A-0289317 describes timber preservative compositions comprising a boron-containing preservative, which may be a micro-crystalline suspension e.g. 10 to 30 microns, and a thickening agent, such as the polysaccharide xanthan gum.
The above preservative compositions were prepared by heating, dissolving and rapid cooling of the ingredients followed by the addition of a thickening agent such as xanthan gum. The resulting compositions are not self-structured as indeed is evidenced by their need always to employ thickening agents.
According to one aspect of the present invention, there is provided a self-structured aqueous sodium pentaborate gel.
It has surprisingly been found that it is possible to provide sodium pentaborate in the form of a self-structured gel, i.e. a gel which does not require thickening agents as suspension aid for the particles. The gels according to the present invention may be readily prepared, are physically stable and can be easily handled. The gels may be pumpable and readily diluted for application as required.
The gels according to the invention comprise sodium pentaborate particles in saturated aqueous solution.
The gels according to the invention can have a high solids contents e.g. up to about 80% by weight, preferably of the order of 60 to 70% by weight. If the solids content becomes too high, the gel may become too stiff for practical use.
The gels according to the present invention may have a high borate content, e.g. 8 to 13% by weight boron as sodium pentaborate but may still be readily handleable, in that the gel is pumpable and can readily be diluted by addition of water. Preferably the gels contain 9 to 12% by weight, most preferably approximately 10% by weight, boron as sodium pentaborate.
The gel according to the invention comprises fine sodium pentaborate particles within a saturated aqueous solution. To produce a self-structured gel, at least a substantial number of the particles must be below 10 microns in size, preferably in the range 0.1 to 10 microns, and more preferably below 5 microns (e.g. 0.1 to 5 microns) in size.
In some circumstances, however, a pourable product having a boron concentration of at least about 12%, may be desired. In these cases, the products of the invention may contain particles the majority of which will still generally be below 10 microns, but the remainder may be larger in size, e.g. up to 50 e.g. 40 microns in size.
The compositions according to the present invention are easy to disperse in aqueous media and have a fast dissolution rate at ambient temperatures and also under field conditions in which the temperature of the water may be as low as 5xc2x0 C., such as may be encountered in agriculture or wood treatment conditions. Moreover, even with high boron contents, the compositions are physically stable and pumpable.
On standing for long periods, there may be some syneresis, i.e. some liquid may separate from the gel. This is unlikely to present any problem as homogeneity can readily be restored by simple mixing.
Syneresis may be reduced by adding an anti-settling agent by techniques well known to those skilled in the art. Anti-settling agents may be selected from high molecular weight polymers, hydroxyethyl cellulose, cross-linked polyacrylates, bentonite gels, silica and combinations of these.
The mechanism of the gel system according to the invention is not fully understood. However it is thought to be caused by the weak attraction of the particles by Van der Waals forces. This is demonstrated by the thixotropic nature of the compositions. Upon shearing the sample and removal of the shear force sol/gel transformation occurs. The gels according to the invention may generally be described as flocculated. Using a rotational viscometer, the flow caurve of shear stress v shear rate, performed in a cycle (whereby the shear rate was increased to 200 secxe2x88x921 and then reduced to 0) showed hysteresis with the reduction in shear (descending) curve being below the increase in shear (ascending) curve.
The gels according to the invention can be stable at temperatures between xe2x88x9210xc2x0 C. and 60xc2x0 C. depending on their composition. The gels can be recovered from the frozen state i.e. they have freeze/thaw stability.
The gel according to the invention may be readily handleable and does not suffer from the disadvantages of fines, dust, caking and segregation which are encountered with solid products. Moreover the pumpable gel according to the invention can be accurately metered.
The gel is self-structured. If desired, the gel may contain additives, for example micronutrient fertilisers, herbicides, fungicides and insecticides.
The borate-containing compositions according to the present invention may be of high density. For example a gel of sodium pentaborate crystals having an equivalent of 10% boron by weight may have a density of the order of 1.3 g/cc.
The gels according to the present invention may be prepared by reacting boric acid or boric oxide and sodium tetraborate or sodium metaborate in suspension in water and under mixing.
According to a second aspect of the present invention, there is provided a method of preparing a self-structured sodium pentaborate gel, which method comprises reacting boric acid or boric oxide and sodium tetraborate or sodium metaborate in suspension in water and under mixing such that there is formed a self-structured sodium pentaborate gel.
The borax and/or sodium metaborate will generally be used in hydrated form, e.g. Na2B4O7. 5 or 10H2O and NaBO2xc2x74H2O.
The starting materials react with the formation of sodium pentaborate precipitate, the particles of which under the influence of the mixing are sufficiently small (usually less than 10 microns) that a self-structured gel is formed.
The starting materials may for example be in dry crystalline form or wet, such as a wet filter cake or wet product obtained following centrifugation in the production of borate, e.g. boric acid or borax, by processes well known in the art. In addition, there may be used plant liquors prom borate production processes. These liquors thus provide the water for the preparation method as well as some of the starting materials. Generally speaking the particle size of the starting materials is not critical.
Three variables which particularly contribute to the production of a self-structured gel are rate of agitation, concentration of the solid ingredients and the temperature used. However, there is a proviso that the solid ingredients should not be allowed fully to dissolve. This allows nucleation to take place readily.
Agitation can be low speed or high speed depending on the solids concentration. Generally the higher the concentration is, the less intense the mixing. Slow agitation tends to give particles having a coarser nature which can contribute to a pourable product.
The temperature will generally be ambient up to near boiling point, e.g. 90xc2x0 C., preferably up to 60xc2x0 C. The use of elevated temperature tends to reduce mixing times.
It is often preferred to use high speed mixing, high concentration and high temperature (normally up to about 60xc2x0 C. but possibly up to 90xc2x0 C.). Low speed mixing generally gives a product with coarser particles and takes longer to form but the product is pourable.
The starting materials are preferably used in stoichiometric amounts for sodium pentaborate. However the molar ratio of B2O3:Na2O may for example be 3:1 to 8:1. The materials used to achieve these ratios would be selected from boric acid, boric oxide, sodium tetraborate or sodium metaborate, with boric acid and borax pentahydrate being preferred. If one of the reactants is in excess, excess unreacted material will generally be present in the resulting gel, and some coarse granules may be retained in the product.
It may be advantageous to carry out the gel formation reaction, in the presence of previously made gel, to act as seed crystals. In this way a significant reduction in the preparation time is achieved. Typically, seed crystals represent from 10% to 50% by weight of the final product.
According to a further aspect of the present invention, there is provided a method of preparing a self-structured sodium pentaborate gel, which method comprises adding agglomerated water-soluble sodium pentaborate particles comprising boric acid, borax pentahydrate and sodium pentaborate to water and mixing such that there is formed a self-structured sodium pentaborate gel. In this method the boric acid and borax pentahydrate react with the sodium pentaborate acting as seed crystals in the production of the gel.
The starting agglomerated particles may be obtained by the process described in WO-A-93/17963. Such particles may be obtained commercially, e.g. under the trade mark SOLUBOR DF. Again the intensity of the mixing and the temperature employed will depend upon each other and the concentration. However the material must not be allowed fully to dissolve.
As mentioned above, it may be advantageous to have some larger crystals present in the gel according to the invention. Such gels can be obtained by the methods described above whereby the reactants are heated to dissolve a part (but not all) of the sodium pentaborate. On cooling some larger crystals (but less than 50, e.g. less than 40, microns in size) are formed.
The gels according to the present invention may be applied, e.g. in the agriculture fields, in timber treatment and in fire retardants, as such or following dilution.
The invention is further illustrated by reference to the following Examples.